Existing closed loop power control algorithms in the Code Division Multiple Access (CDMA) and Universal Mobile Telephone System (UMTS) ignore the roundtrip delay incurred in the measurement of the received signal power. Such an assumption of zero round trip delay may be valid when the round trip delay is negligible in comparison to the inverse of the channel fading bandwidth. However, in the case of satellite channels such a condition is not satisfied. The round trip delay for geostationary satellites, for example, may be in the range of 250 msec to 500 msec, including any terrestrial delay, for a single hop or two-hop system. When compared to the inverse of the channel fading delay in the range of 1.0 to 10.0 sec, corresponding to the bandwidth in the range of 0.1 to 1.0 Hz, such a delay is not negligible. Hence, any power control algorithm based on such a zero delay assumption will not have satisfactory performance. Power control is necessary for frequency division multiple access (FDMA) and time division multiple access (TDMA) systems to control adjacent channel interference and mitigate unexpected interference caused by a near-far problem. In these systems, power control is needed to improve the quality of voice delivered to a user. However, in CDMA systems that are self-interfering, the capacity of the system depends directly on the power control and an accurate power control mechanism is needed for maximizing the number of users that can operate simultaneously in the system. Improved power control can also save the amount of transmitted power of the mobile station (MS), which increases the life of the battery.
In case of fading channels, diversity-combining techniques are generally employed to improve system performance. For example, in case of the CDMA systems, implicit diversity gain is exploited with the help of a rake receiver. In such a diversity system employing either an explicit or an implicit diversity, the power control in the terrestrial systems is based on the estimated power level at the diversity combiner output. In CDMA systems the power control on the reverse link, for example, is based on a combination of open loop control along with a closed loop correction implemented in the base station and the MS. For open loop probing on the access channel with closed loop correction inactive, the mobile station transmits the first probe signal at an output power level, defined by a mean output power in dBm is equal to k minus the mean receive input power in dBm plus 0.5, times, the nominal power in dB plus 0.5, times, the initial power in dB. The mean power is referred to a nominal CDMA channel bandwidth of 1.23 MHz and k, nominal power, and initial power are system parameters. The initial power parameter is any margin, positive or negative, above the required level. When the initial power is zero, then the nominal power is the correction to provide the correct received power at the base station. Essentially the power control, in both open loop and closed loop, is based on the assumption of identical channel gain on both the reverse and forward link, thus ignoring both the roundtrip propagation delay and the difference in the transmission frequency on the two links. Subsequent probes in an access probe sequence are sent at an increasing power level until a response is obtained from the base station. The initial transmitted power in the reverse traffic channel is at the power level given by the mean TX output power equation plus all the access probe corrections.
Uncontrolled differences in the forward and reverse channels, such as opposite fading that may result due to the frequency difference plus mismatches in the mobile station's transmit and receive chains result in the transmit power, which is different than the desired level. To reduce these residual errors, a close loop control is used wherein the mobile station measures the received Eb/N0 which is the bit energy to total noise power spectral density ratio, and transmits this information to the base station on the reverse link. The base station compares the performance measure against a threshold and based on the difference between the two sends a correction signal to the mobile station inserted in the forward data steam. The MS then corrects his transmit power on the basis of this correction information.
The closed loop correction in the CDMA systems ignores the round trip delay in comparison to the inverse of the channel fading bandwidth. For example, typically the fading bandwidth for the terrestrial cellular communication networks may be of the order of 50.0 Hz having a time constant of 20.0 msec compared to a round trip delay smaller than 0.1 msec for a cell radius of 10 Km and thus such assumption is satisfied. However, in the case of satellite channels with a roundtrip delay of 500 to 600 msec such an assumption is not valid even when the fading bandwidth is less than 1 Hz.
In the UMTS system, two loops for power control are involved. The inner loop is based on a bandwidth of about 1500 Hz with a period of 0.66 msec, measures the received signal to interference ratio and compares to the desired signal to interference ratio. The loop period is much higher compared to the roundtrip delay and thus the roundtrip delay is not too important in the loop design. This loop is similar to the closed loop in the CDMA system. An outer loop measures a service metric such as the frame error rate and adjusts the desire signal to interference ratio to account for any unmodeled uncertainties. The outer loop bandwidth is in the range of 10.0 to 100.0 Hz. Similar to the CDMA power control, a satisfactory operation of the UMTS system power control requires that the roundtrip delay be negligible compared to the channel fading bandwidth and the loop bandwidth. However, in the case of satellite channels such a condition is not satisfied. In case of the satellite channels, the round trip delay may vary between 250 to 550 msec, depending upon whether a single hop or two hop system is used, compared to 1.0 μsec to 100 μsec variation in UMTS system. When compared to a fading channel bandwidth of 0.1 to 1.0 Hz with a time constant of 1.0 to 10.0 seconds, the roundtrip delay is not negligible and hence the power control approach will not work satisfactorily. It is therefore necessary to take into account the roundtrip propagation delay explicitly in the design of the power control algorithm. A satisfactory operation of both the CDMA and the UMTS power control algorithm requires that the roundtrip delay be negligible compared to the channel fading bandwidth and the loop bandwidth. When compared to a fading channel bandwidth and time constant, the roundtrip delay is not negligible and hence the prior power control algorithms may not work satisfactorily. These and other disadvantages are solved or reduced using the invention.